To reproduce:

Sub New()

    InitializeComponent()

    Dim dt As New DataTable()
    dt.Columns.Add("Price", GetType(System.Double))
    dt.Columns.Add("Name", GetType(System.String))
    dt.Columns.Add("Nr", GetType(System.Double))
    For i As Integer = 0 To 49
        dt.Rows.Add(i, "Data" & i, i)
    Next
    With Me.RadGridView1
        .DataSource = dt
        .Columns("Price").FormatString = "{0:C2}"
        .Columns("Nr").FormatString = "{0:N1}"
        .Columns("Price").ExcelExportType = Export.DisplayFormatType.Currency
        '
        .AutoSizeColumnsMode = Telerik.WinControls.UI.GridViewAutoSizeColumnsMode.Fill
    End With
    Me.RadGridView1.Columns("Price").ExcelExportType = Export.DisplayFormatType.Currency

    Me.RadGridView1.Columns("Nr").ExcelExportType = Export.DisplayFormatType.Custom
    Me.RadGridView1.Columns("Nr").ExcelExportFormatString = "{0:N1}"

    Me.RadGridView1.AutoSizeColumnsMode = Telerik.WinControls.UI.GridViewAutoSizeColumnsMode.Fill

End Sub

Private Sub RadButton1_Click(sender As Object, e As EventArgs) Handles RadButton1.Click
    Dim spreadStreamExport As New GridViewSpreadStreamExport(Me.RadGridView1)
    spreadStreamExport.ExportVisualSettings = True
    Dim fileName As String = "..\..\" + DateTime.Now.ToLongTimeString().Replace(":", "_").ToString() + ".xlsx"
  
    spreadStreamExport.RunExport(fileName, New SpreadStreamExportRenderer())
    Process.Start(fileName)
End Sub

Workaround:
Me.RadGridView1.Columns("Nr").ExcelExportType = Export.DisplayFormatType.Fixed

AddHandler spreadStreamExport.CellFormatting, AddressOf CellFormatting
Private Sub CellFormatting(sender As Object, e As SpreadStreamCellFormattingEventArgs)
        If e.ExportCell.ColumnIndex = 2 Then
            e.ExportCell.ExportFormat = "0.0"
        End If
    End Sub